Conventionally, an attraction object to be transported by an attracting transport arm is detected using, for example, a pressure sensor or a transmission type sensor.
Therefore, an attraction object detection apparatus which employs a pressure sensor is shown in (a) and (b) of FIG. 1 while another attraction object detection apparatus which employs a transmission type sensor is shown in FIG. 2, and their constructions and operations are described.
The apparatus shown in FIG. 1 is constructed such that it includes an arm 2 having a suction opening and movable in a horizontal direction and a vertical direction and air around the suction opening of the arm 2 is sucked into a vacuum source (not shown) via a pressure sensor 3. In the present apparatus, whether or not an attraction object 1 is present is discriminated by a pressure sensor 3 when the arm 2 is moved down to a location at which the attraction object 1 is to be placed. In particular, if the attraction object 1 is not present when the arm 2 is moved down, since the suction opening of the arm 2 remains in a leaking condition, a drop of pressure is not detected by the pressure sensor 3, but if the attraction object 1 is present, then there is no leak from the arm 2 and a drop of pressure is detected by the pressure sensor 3. From this, whether or not the attraction object 1 is present can be discriminated.
Meanwhile, in the apparatus shown in FIG. 2, a transmission type sensor is disposed at a location at which an attraction object 1 is to be placed. In the transmission type sensor, rays of light which go across the location at which the attraction object 1 is to be placed are formed from a pair of light emitting elements 4 and a pair of light receiving elements 5. In the apparatus of the type just described, whether or not an attraction object 1 is present can be discriminated as the attraction object 1 intercepts rays of light between the pair of light emitting elements 4 and the pair light receiving elements 5 of the transmission type sensor.
However, such an apparatus as shown in FIG. 1 is disadvantageous in that whether or not an attraction object is present cannot be discriminated unless the arm is moved down to the location at which the attraction object is to be placed. The apparatus is disadvantageous also in that it may possibly discriminate in error that an attraction object is present because of an error in adjustment of the pressure sensor.
On the other hand, such an apparatus as shown in FIG. 2 is disadvantageous in that it cannot be used where the sensor cannot be mounted in horizontal and oblique directions with respect to an attraction object. The apparatus is disadvantageous also in that, where it is intended to discriminate whether or not an attraction object is present at each of a large number of attraction points, a large number of sensors are required.